Reduced-order Dynamic Modeling and Robust Nonlinear Control of Fluid Flow Velocity Fields

A robust nonlinear control method is developed for fluid flow velocity tracking, which formally addresses the inherent challenges in practical implementation of closed-loop active flow control systems. A key challenge being addressed here is flow control design to compensate for model parameter variations that can arise from actuator perturbations. The control design is based on a detailed reduced-order model of the actuated flow dynamics, which is rigorously derived to incorporate the inherent time-varying uncertainty in the both the model parameters and the actuator dynamics. To the best of the authors’ knowledge, this is the first robust nonlinear closed-loop active flow control result to prove exponential tracking control of a reduced-order actuated flow dynamic model, which formally incorporates input-multiplicative time-varying parametric uncertainty and nonlinear coupling between the state and control signal. A rigorous Lyapunov-based stability analysis is utilized to prove semiglobal exponential tracking of a desired flow field velocity profile over a given spatial domain. A detailed comparative numerical study is provided, which demonstrates the performance improvement that is achieved using the proposed robust nonlinear flow control method to compensate for model uncertainty and uncertain actuator dynamics

On the estimation of time dependent lift of a European Starling during flapping

We study the role of unsteady lift in the context of flapping wings in birds' flight. Both aerodynamicists and biologists attempt to address this subject, yet it seems that the contribution of the unsteady lift still holds many open questions. The current study deals with the estimation of unsteady aerodynamic forces on a freely flying bird through analysis of wingbeat kinematics and near wake flow measurements using time resolved particle image velocimetry. The aerodynamic forces are obtained through unsteady thin airfoil theory and lift calculation using the momentum equation for viscous flows. The unsteady lift is comprised of circulatory and non-circulatory components. Both are presented over wingbeat cycles. Using long sampling data, several wingbeat cycles have been analyzed in order to cover the downstroke and upstroke phases. It appears that the lift varies over the wingbeat cycle emphasizing its contribution to the total lift and its role in power estimations. It is suggested that the circulatory lift component cannot assumed to be negligible and should be considered when estimating lift or power of birds in flapping motion

On Progress in Exploring Controlled Viscous Limit-Cycle Oscillations in Modified Glauert Airfoil

The paper reports on the progress in the development of a novel robust, nonlinear flow control technology that employs an array of synthetic-jet actuators (SJAs) embedded in 2-DOF, elastically mounted, optimized Modified Glauert (MG) airfoil design in order to control limit cycle oscillations (LCO) at low subsonic flow regimes. The focus here is on the conceptual design of the wind energy harvesting system that employs, e.g., a piezoelectric device to extract energy from plunging LCO, with the closed-loop controller being capable to sustain the required LCO amplitudes over a wide range of wind speeds. The current high-fidelity studies first include validation of the static-airfoil aerodynamic predictions against results obtained from the concurrent experimental campaign. Next, a set of parametric 1-DOF and 2-DOF numerical analyses examine open-loop and closed-loop LCO control strategies that employ the ability of MG airfoil to sustain LCO at subcritical velocities due to natural separation-induced flutter

Unsteady aerodynamics of single and tandem wheels

The major unsteady aerodynamic forces and major physics of a generic single wheel and tandem wheels are studied for the first time using wind tunnel tests. The wind-tunnel tests are performed in the 2.1 m × 1.5 m wind tunnel at the University of Southampton. The tandem-wheel configuration consists of two in-line wheels that can be tested at different inter-axis distances and various installation angles. A vibration test is performed in situ on the model assembly to validate the unsteady-load measurements. Mean and unsteady aerodynamic loads and on-surface pressures are measured. Particle Image Velocimetry is used to acquire the velocity fields in the wake downstream of the model and surface oil-flow technique is used to identify the flow features on the surface of the wheels. Proper Orthogonal Decomposition is also used to characterise the wake in terms of unsteady fluctuations. The results of the experiments on the tandem wheels show that higher values of inter-axis distance correspond to slightly higher total mean drag coefficients and remarkably lower drag coefficient RMS values. Higher installation angles are associated with higher mean drag coefficients but generally lower fluctuations of the force coefficients. Non-zero mean lift coefficients are found for low inter-axis distance configurations at zero installation angle. The flow on the single wheel and on the front wheel of the tandem wheels is affected by laminar-turbulent transitional features. The vortical structures past the tandem wheels consist of four vortices that detach from the tyre shoulders of the front wheel and interact with the rear wheel. The study and obtained databases contribute to the general understanding of the complex flow and help to improve engineering predication of the gear aerodynamic loads

Unsteady force and flow features of single and tandem wheels

Wind-tunnel experiments are presented in this paper for two different models, single wheel and tandem wheels. The tests are performed in the 2.1 m × 1.5 m wind tunnel at the University of Southampton. The aims of the experiment are to gain a better understanding of the flow past simple landing-gear components and to generate a CFD validation database. Since the model is designed to study basic landing-gear components, the wheel geometry is simplified, with no detailed elements in the assembly. The tandem-wheel configuration is formed of two in-line wheels that can be tested at different inter-axis distances and various angles of attack. Mean and unsteady data of aerodynamic loads and on-surface pressures are measured. A vibration test is performed in situ on the model assembly to validate the unsteady-load measurements. Particle Image Velocimetry (PIV) is used to acquire the velocity fields in the wake downstream of the model. The results highlight the low sensitivity of the measured quantities to the three versions of the wheel hub on the single wheel. The mean drag coefficients of the tandem wheels show a low sensitivity to the inter-axis distance, which has stronger effects on the mean lift coefficients and the unsteady aerodynamic loads. The angle of attack determines relevant changes in both mean and unsteady quantities. The pressures on the wheel surface are used for gaining a better understanding of the flow regimes and the effect of tripping the flow. Additionally, the PIV data are used to compare the velocity profiles in the wake and identify the wake vortical structures.<br/

Towards a non-empirical trailing edge noise prediction model

This paper extends previously published TNO-Blake model to predict airfoil broadband self-noise due to the interaction of a turbulent boundary layer with a sharp trailing edge. The main objectives of this paper are reduced the dependence on ‘turning parameters’. The method presented herein combines an extension to Blake׳s model to predict the pressure fluctuation on the airfoil surface due to the turbulent boundary layer. Flat plate theory, with finite-chord effects included, is introduced to predict its subsequent radiation to the far field. The paper builds on recent advances in the prediction of the streamwise turbulent intensity profile to avoid the use of empirical relationships. Surface pressure spectra measurements are compared to predictions where agreement within 2 dB is obtained in the mid to high frequency range

Leading Edge Blowing to Mimic and Enhance the Serration Effects for Aerofoil

Leading edge serration is now a well-established and effective passive control device for the reduction of turbulence–leading edge interaction noise, and for the suppression of boundary layer separation at high angle of attack. It is envisaged that leading edge blowing could produce the same mechanisms as those produced by a serrated leading edge to enhance the aeroacoustics and aerodynamic performances of aerofoil. Aeroacoustically, injection of mass airflow from the leading edge (against the incoming turbulent flow) can be an effective mechanism to decrease the turbulence intensity, and/or alter the stagnation point. According to classical theory on the aerofoil leading edge noise, there is a potential for the leading edge blowing to reduce the level of turbulence–leading edge interaction noise radiation. Aerodynamically, after the mixing between the injected air and the incoming flow, a shear instability is likely to be triggered owing to the different flow directions. The resulting vortical flow will then propagate along the main flow direction across the aerofoil surface. These vortical flows generated indirectly owing to the leading edge blowing could also be effective to mitigate boundary layer separation at high angle of attack. The objectives of this paper are to validate these hypotheses, and combine the serration and blowing together on the leading edge to harvest further improvement on the aeroacoustics and aerodynamic performances. Results presented in this paper strongly indicate that leading edge blowing, which is an active flow control method, can indeed mimic and even enhance the bio-inspired leading edge serration effectively

Unsteady aerodynamics of single and tandem wheels

The major unsteady aerodynamic forces and major physics of a generic single wheel and tandem wheels are studied for the first time using wind tunnel tests. The wind-tunnel tests are performed in the 2.1 m × 1.5 m wind tunnel at the University of Southampton. The tandem-wheel configuration consists of two in-line wheels that can be tested at different inter-axis distances and various installation angles. A vibration test is performed in situ on the model assembly to validate the unsteady-load measurements. Mean and unsteady aerodynamic loads and on-surface pressures are measured. Particle Image Velocimetry is used to acquire the velocity fields in the wake downstream of the model and surface oil-flow technique is used to identify the flow features on the surface of the wheels. Proper Orthogonal Decomposition is also used to characterise the wake in terms of unsteady fluctuations. The results of the experiments on the tandem wheels show that higher values of inter-axis distance correspond to slightly higher total mean drag coefficients and remarkably lower drag coefficient RMS values. Higher installation angles are associated with higher mean drag coefficients but generally lower fluctuations of the force coefficients. Non-zero mean lift coefficients are found for low inter-axis distance configurations at zero installation angle. The flow on the single wheel and on the front wheel of the tandem wheels is affected by laminar-turbulent transitional features. The vortical structures past the tandem wheels consist of four vortices that detach from the tyre shoulders of the front wheel and interact with the rear wheel. The study and obtained databases contribute to the general understanding of the complex flow and help to improve engineering predication of the gear aerodynamic loads